The aim in causticizing is to react lime exiting a lime sludge reburning kiln with green liquor exiting a soda recovery boiler in appropriate circumstances and in a suitable mixture ratio to produce white liquor of a desired quality and lime mud, i.e. calcium carbonate CaCO3. The causticizing process can be divided into three phases: slaking, causticizing and white liquor preparation. The most important phase in the process is the feeding of lime into the slaker/screening unit where the lime is slaked, and the causticizing reaction begins. The causticizing process is therefore typically controlled by controlling the operation of the slaker, which is currently based on controlling the lime to green liquor ratio and the difference between the temperature in the slaker and that of the green liquor. The ratio control aims at maintaining a correct lime feed level in all circumstances, and the temperature difference control is used to compensating for changes in lime quality or quantity by changing their ratio as the temperature difference changes. This is based on the exothermicity of the slaking reaction: changes in lime quality or quantity are manifested by changes in the temperature difference between the slaker and the green liquor entering the slaker. However, the ratio or the temperature difference does not tell the real state of the process.
Various automated titrators have been used to further improve the control. A titrator provides reliable information about the composition of green liquor and lime milk at intervals of about 10 minutes. The results allow the settings of the temperature difference control and the ratio control to be changed to obtain optimal end product quality. The most commonly used variable in the control is white liquor causticity, i.e. conversion of sodium carbonate to sodium hydroxide, which is an active compound in pulping.
The ratio control is a rough adjustment connected to operate in a feedforward manner with respect to a green liquor flow rate controller. In some control solutions, analysis results provided by the automated titrator are applied to directly influence the set value of the ratio control on the basis of a mathematical formula that takes into account aspects such as the causticity prevailing after the slaker and the last causticizing vessel, green liquor quality, etc. The change in the set value of the ratio control is always calculated after the titration has been completed.
Temperature difference control arranged in addition to ratio control provides an end product of a more uniform quality, because in a short run changes in lime quality or quantity can be corrected by influencing the ratio control. An automated titrator and a temperature difference control allow a discontinuous absolute measurement to be combined with continuous relative measurement. In previous solutions attempts have been made to apply a mathematical formula to the results provided by the titrator and the measurements of green liquor and the slaker to calculate a suitable temperature difference change that would allow a desired lime milk causticity and/or white liquor causticity to be obtained after the slaker. The change with respect to the set value of the temperature difference is always calculated after the titration has been completed. A common aspect of these known control methods is that the causticizing degree achieved after the slaker is kept constant during the entire control process.
However, a major problem in the slaker control is the variations taking place in the green liquor temperature and density despite the control. Changes in green liquor density, and thereby in the total titratable alkali TTA, as well as temperature changes cause disturbances in the slaker control. A change in the TTA affects the chemical balance of the causticizing process directly; it changes the kinetics of the process and, together with sulphidity, it determines at the same time a theoretical maximum for causticity. Temperature changes in green liquor distort the information supplied to temperature difference control, because in the slaker temperature the changes appear both as delayed and filtered. In other words, they are incorrectly manifested as quality changes to the temperature difference control.
Sometimes the physical and chemical quality of lime is such that despite the temperature difference control and a uniform lime feed, great fluctuations appear in temperature differences and, thereby, in causticities. Changes in lime quality emerge in the lime sludge reburning kiln, or when lime mud is fed into a lime silo. Production changes, raw material variations, mechanical disturbances, etc., affecting the lime sludge reburning kiln change the granular size of lime and both its physical and chemical structure, which in turn changes its slaking properties, caustic efficiency and flow properties. The degree of filling of the lime silo affects the packing, temperature and behaviour of lime in the feed screw. When all the lime has been spent, or fresh, reactive lime is introduced, the temperature difference in the slaker and the causticities change radically in a short period of time.
Problems also arise from lime feed: for example, the scaffolding of the silo and the wearing of the feed screw cause disturbances in lime feed which cannot be corrected with conventional methods, either due to lack of time or efficiency. Wearing of and disturbances in the lime feed equipment, as well as changes in lime quality, cause instability, disturbances and hysteresis in the control settings which is shown in that the change in the temperature difference in response to a change in the lime to green liquor ratio is delayed. The delay, in turn, causes oscillation in the control, thereby degrading the quality of the end product.
Problems are also caused by the interval of automated titrations; the minimum interval is 10 minutes and, in practice, the interval for the most significant measurements is over 20 minutes. In addition, the completion of the titration after the sampling takes several minutes. Long titration intervals retard the total control process because the temperature difference control settings can only be changed after the titration is completed.
Further problems are caused by process delays. For example, the minimum time from the lime feed to the titration taking place after the slaker is half an hour, the titration performed after the last causticizing vessel consuming typically 3 to 4 hours. Delays are known to cause problems of control, because the longer the time from the measured disturbances and changes, the more difficult it is to influence them.
Long and changing delays, changes in green liquor quality and in lime quality and quantity make it almost impossible to use both the causticity after the slaker and that after the last causticizing vessel for active control. Known methods have attempted to take into account measured and titrated variable values and changes in them when calculating changes in either the temperature difference control or the ratio control.
A basic problem involved in a control system employing a titrator and based on the ratio control alone is that it does not take into account changes taking place between the titrations at all. Depending on the condition of the titrator, measurements have to be filtered, or even rejected, if they deviate too much from the previous titrations. Nevertheless, it is not certain whether an individual result is a real one or whether it includes a deviation caused by process conditions or the titrator. Moreover, the control process must be made slow, because great variations in set values cause a discretely adjusted process to oscillate easily. Since the process is affected by transit time delays and measurement delays, a discrete control employed alone will ultimately lead to a situation where the measurement deviates from the set value to such an extent that the correction is either too slow, or it causes the process to oscillate on both sides of the set value. In both cases, quality is impaired and production is lost due to low causticity or the blocking of the white liquor filter. In addition to this, the observations relating to lime feed and causticity restrictions described below are valid.
In applications employing a titrator and based on temperature difference, the discreteness of control has been eliminated because the temperature difference shows even major changes taking place at short intervals quickly and in real-time. There are still a few aspects that have not been taken into account in known solutions, for example problems related to lime feed and the correlation between production phase and causticities. Furthermore, these solutions lack the actual dynamics that allow the control system to be active almost in every situation without the operator being required to intervene in the set values or the state of the control system. One example is applications where the causticity set value must be changed every time production changes.
WO publication 98/10137 discloses a solution for controlling the causticizing process by calculating the causticizing degree to control lime feed. The proportions of the different green and white liquor components are measured, and the causticizing process is controlled using for example a neural network or fuzzy logic. The solution is fairly complex and does not produce a sufficiently good end result for the control in all respects.
U.S. Pat. No. 5,378,320 discloses a solution where samples are taken from the causticizing process, the properties of the samples being determined by applying infrared spectrophotometry. The measurement results are used for controlling the amount of lime. Also this solution does not provide a sufficiently good causticizing process control in all respects.
FI Patent 66,662 discloses a solution for controlling the causticizing degree by taking samples from the liquid (white liquor) exiting the slaker and the liquid (green liquor) fed to the slaker. The causticizing degree is controlled on the basis of the carbonate-ion content of these samples. The carbonate-ion contents are determined using a specific analyzer where the carbonate included in the sample is converted to carbon dioxide, the amount of which is then measured. This allows the amount of carbonate included in the sample to be concluded and, thereby, the amount of calcium oxide to be added to the causticizing process to be controlled. The patent also describes factory tests where the concentration of the green liquor, i.e. its carbonate content, is first monitored by applying density determination to provide comparison data, then calcium oxide is added, and finally the above mentioned analyzer is used for determining the carbonate-ion contents of both the green and the white liquor. The amount of calcium oxide added is controlled on the basis of the measured carbonate-ion contents, which allows the desired causticizing degree to be achieved. However, all in all the solution of the invention does not allow the causticizing process to be controlled in a satisfactory manner.
FI Patent 76,137 discloses a method for controlling white liquor properties by measuring the electrical conductivity of green liquor and by determining a TTA value for example by measuring the specific weight, or the absorption of gamma radiation, of the green liquor. The use of an electrical conductivity meter involves several drawbacks. For example, the measurement of electrical conductivity is often very inaccurate because temperature and electrical and other disturbances affect the measurement result to a considerable extent. Furthermore, the equipment must be calibrated quite often. Therefore, also in this case the causticizing process cannot be controlled in an entirely satisfactory manner.
It is an object of the present invention to provide a method and an apparatus for eliminating at least some of the above problems.